1. Field of the Invention
The present invention relates to an optical information processing apparatus for irradiating a focused light beam to an optical recording medium to record and/or reproduce information.
2. Related Background Art
An apparatus for recording and reproducing a signal by utilizing a laser beam has been put in practice. Examples are a compact disk (CD) player and a laser disk (LD) player. In such an apparatus, a signal is recorded as pits 2 on a disk 1 as shown in FIG. 1. In order to read the information signal from the disk 1, a light beam is irradiated to the disk 1 and it is optically scanned to read the pits 2. By scanning a track in which the pits 2 have been formed, by the light beam, the strength of reflected light changes depending on the presence or absence of a pit so that the information pits can be optically read out.
On the other hand, a magneto-optical disk apparatus in which a light beam is irradiated to a magneto-optical disk having spiral or concentric tracks to optically record, reproduce and erase an information signal, has also been developed. FIG. 2 shows a magneto-optical disk 3 used in such an apparatus.
In FIG. 2, numeral 4 denotes a signal track guide groove. A magneto-optical signal is recorded at a mid-point of the track guide groove and the adjacent track guide groove. Numeral 5 denotes preformat pits of a detection pattern such as track address, sector address and sector mark formed between adjacent track guide grooves when the magneto-optical disk is to be used as a data information disk. An address is detected from the preformat pits 5, and data corresponding to the address are reproduced, recorded or erased by the magneto-optical signal before the next preformat pit 5 is detected.
In the tracking servo system in which tracking is effected by using the track guide groove 4 of the disk 3, a reflection factor of the disk surface is low at the position of the preformat pit, and an S/N ratio of the tracking error signal is also low, and noise appears. While the preformat pits occupy only 4-5% of the total track gruide grooves, they lower the reliability of tracking and make the servo system unstable.
On the other hand, Japanese Laid-Open Patent Application No. 59-142757 discloses an apparatus which detects a defect of an optical recording medium to hold a tracking signal. However, in this apparatus, no countermeasure for the impact of the preformat signal to the servo system is taken.
The above problem also occurs in a focusing servo for accurately focusing the light beam onto the medium.